Mg alloy, method for manufacturing mg alloy, and construction material and biomaterial using mg alloy

ABSTRACT

An Mg alloy is provided the degradation of which is promoted by forming and crystallizing an Al—Mn—Ni system intermetallic compound containing Ni together with the metals included in a magnesium alloy, and thereby dispersing Ni in the magnesium alloy.

TECHNICAL FIELD

The present invention relates to an Mg alloy, a method for manufacturing an Mg alloy, and a construction material and a biomaterial using an Mg alloy. More particularly, it relates to an Mg alloy capable of promoting degradation.

BACKGROUND ART

A magnesium alloy is smaller in density among the metal materials for use in a structure or a device. For this reason, the members in various fields are replaced with a magnesium alloy from iron or the like, thereby reducing weight of the members. Further, a magnesium alloy is more base in electric potential than other metals, and hence is also applied to a sacrificial electrode material or a drilling material for preventing corrosion of the structure. Further, a magnesium alloy has degradability or biodegradability, and hence is also used for a member not required to be collected. Application and development thereof to an underground structure, an underwater structure, a biomaterial, and a medical material have also been progressed.

PTL 1 relates to an MgZn alloy or an MgZnCa alloy having an improved degradation characteristic, and discloses an implant having a three-dimensional structure based on the alloys. The implant is a material for medical use including a surgical implant, and hence, specifically, an ultra-high purity magnesium is allowed to contain high purity Zn in an amount of 2.0 wt % to 6 wt % (PTL 1 paragraphs [0002], [0004], [0045], and the like).

Further, PTL 2 relates to a magnesium alloy material excellent in mechanical characteristics, and surface quality, and discloses that, for performing continuous casting, the forming material for the portion to be in contact with the molten metal of a magnesium alloy is formed of a low oxygen material with an oxygen content of 20 mass % or less (PTL 2 paragraphs [0008], [0009], and the like).

As described above, a material for a magnesium alloy excellent in weight reduction, mechanical characteristics, and degradation characteristic has been developed according to the intended purpose.

On the other hand, commercially available magnesium includes impurities present therein. It is considered that the presence of such impurities enhances the degradation rate due to formation of micro galvanic elements including Fe, Cu, and Ni (PTL 1 paragraphs [0004] and the like). In other words, Ni has a property of enhancing the degradation rate, and is considered to be able to adjust the degradation rate depending upon the state of existence in the magnesium alloy. However, Ni has higher melting point and density than those of Mg or a magnesium alloy (the melting point of Mg is 650° C., the density of Mg is 1.738 g/cm³, the melting point of Ni is 1455° C., and the density of Ni is 8.908 g/cm³). For this reason, unfavorably, it is difficult to add NI to a magnesium alloy for dissolution or complete dispersion in the alloy within the temperature region in which the magnesium alloy is molten.

Further, as described above, it is difficult to add Ni to a magnesium alloy for dissolution or complete dispersion in the alloy. For this reason, even when Ni having a property of enhancing the degradation rate is simply added into a magnesium alloy, unfavorably, it is difficult to implement an Mg alloy capable of promoting degradation in line with the intention.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Translation of PCT Application Publication No.     2015-532685 -   [PTL 2] WO 2006/003899

SUMMARY OF INVENTION Technical Problem to be Solved by Invention

The present invention was completed in view of the foregoing circumstances. It is an object of the present invention to provide an Mg alloy including Ni dispersed in a magnesium alloy together with metals contained in the magnesium alloy.

Solution to Problem

In order to solve the problem, an Mg alloy of the present invention contains Mg, Al, Mn, and Ni, and has a crystallized Al—Mn—Ni system intermetallic compound.

The Mg alloy may further contain Zn.

The Mg alloy may further contain Ca.

The Mg alloy containing Ca may contain one or more compounds selected from a group consisting of Al₂Ca, (Mg, Al)₂Ca, and Mg₂Ca.

The Mg alloy is preferably configured such that the Al is in an amount of 0.1 mass % or more, and the Mn is in an amount of 0.05 mass % or more based on a total amount of the Mg alloy.

The Mg alloy containing Zn is preferably configured such that the Zn is in an amount of 0.05 mass % or more and 1.5 mass % or less based on a total amount of the Mg alloy.

The Mg alloy containing Ca is preferably configured such that the Ca is in an amount of 0.1 mass % or more and 2.0 mass % or less based on a total amount of the Mg alloy.

It is preferable that Ni is in an amount of 0.1 mass % or more based on an amount of the Al—Mn—Ni system intermetallic compound.

In the Mg alloy, preferably, the Al—Mn—Ni system intermetallic compounds are at one compound/cm² or more per unit sectional area, and/or each have a size of 1 nm or more and 25 μm or less.

The Al—Mn—Ni system intermetallic compound may form clusters.

A method for manufacturing an Mg alloy having a crystallized Al—Mn—Ni system intermetallic compound includes casting. The casting includes a step of producing a mixture by blending Mg, Al, Mn, and Ni; a step of producing a molten metal by heating the produced mixture to 720° C. or more; a step of producing a completely molten metal by stirring the produced molten metal; and a step of casting the completely molten metal produced by stirring.

The step of producing the mixture further includes blending Zn and/or Ca.

The construction material or the biomaterial of the present invention uses the Mg alloy, and is not required to be collected after use due to the degradability of the Mg alloy.

Advantageous Effects of the Invention

The Mg alloy containing Mg, Al, Mn, and Ni of the present invention has a crystallized Al—Mn—Ni system intermetallic compound. This can provide an Mg alloy capable of promoting degradation.

Further, with the method for manufacturing an Mg alloy of the present invention, an Al—Mn—Ni system intermetallic compound containing Ni together with Al and Mn of metals contained in the magnesium alloy can be formed and crystallized, and Ni can be dispersed in the magnesium alloy, which can manufacture an Mg alloy capable of promoting degradation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows one example of a manufacturing process of the present invention.

FIG. 2 shows one example of the relationship between the ratio of the content to the Ni addition amount and the molten metal temperature of the present invention.

FIG. 3(a) shows one example of the kind of a crystal according to whether stirring is performed or not in the casting step. FIG. 3(a) is an example without stirring.

FIG. 3(b) shows one example of the kind of a crystal according to whether stirring is performed or not in the casting step. FIG. 3(b) shows an example with stirring.

FIG. 4(a) shows one example of the casting step of the present invention. FIG. 4(a) is one example of the relationship between the elapsed time and the temperature of the casting step.

FIG. 4(b) shows one example of the casting step of the present invention. FIG. 4(b) is a metal microphotograph of a billet casted in FIG. 4(a).

FIG. 4(c) shows one example of the casting step of the present invention. FIG. 4(c) is a metal microphotograph on an enlarged scale of FIG. 4(b).

FIG. 5(a) shows one example of a metal microphotograph of an Mg alloy based on the Ni addition amount. FIG. 5(a) is an example of the case where the Ni addition amount of 0.4 mass %.

FIG. 5(b) shows one example of a metal microphotograph of an Mg alloy based on the Ni addition amount. FIG. 5(b) is an example of the case where the Ni addition amount is 5 mass %.

FIG. 6(a) shows a metal microphotograph of an Al—Mn—Ni system intermetallic compound of a billet subjected to a homogenization treatment. FIG. 6(a) shows an example of a visual field 1.

FIG. 6(b) shows a metal microphotograph of an Al—Mn—Ni system intermetallic compound of a billet subjected to a homogenization treatment. FIG. 6(b) shows an example of a visual field 2.

FIG. 6(c) shows a metal microphotograph of an Al—Mn—Ni system intermetallic compound of a billet subjected to a homogenization treatment. FIG. 6(c) shows an example of a visual field 3.

FIG. 7(a) shows a metal microphotograph of an Al—Mn—Ni system intermetallic compound of an extruded material. FIG. 7(a) shows an example of a visual field 1.

FIG. 7(b) shows a metal microphotograph of an Al—Mn—Ni system intermetallic compound of an extruded material. FIG. 7(b) shows an example of the visual field 2.

FIG. 8 shows one example of the relationship between the Ni density and the degradation rate in the Mg alloy of the present invention by whether Ca is added or not.

FIG. 9(a) shows an example of the SEM-EDS analysis result when Ca is added in the Mg alloy of the present invention. FIG. 9(a) shows an example of the visual field 1.

FIG. 9(b) shows an example of the SEM-EDS analysis result when Ca is added in the Mg alloy of the present invention. FIG. 9(b) shows an example of the visual field 2.

FIG. 10 shows an example of the relationship among the density of Ca added, the characteristics of the tensile rupture strength, the 0.2% proof strength, and the elongation.

FIG. 11(a) shows one example of the degradation mechanism study. FIG. 11(a) shows the extrusion direction and the observation direction of a sample.

FIG. 11(b) shows one example of the degradation mechanism study. FIG. 11(b) shows an example of a metal microphotograph before the immersion test of the extruded material.

FIG. 11(c) shows one example of the degradation mechanism study. FIG. 11(c) shows an example of a metal microphotograph after the immersion test of the extruded material.

FIG. 12(a) shows one example in which the Al—Mn—Ni system intermetallic compound becomes clusters in the Mg alloy of the present invention. FIG. 12(a) shows one example of a cluster forming sample.

FIG. 12(b) shows one example in which the Al—Mn—Ni system intermetallic compound becomes clusters in the Mg alloy of the present invention. FIG. 12(b) shows another example of a cluster forming sample.

DESCRIPTION OF EMBODIMENTS

Below, an aspect for executing the present invention will be described.

The Mg alloy of the present invention contains Mg, Al, Mn, and Ni, and has a crystallized Al—Mn—Ni system intermetallic compound.

(Mg Alloy)

An Mg alloy is literally an alloy containing Mg as the main component. For the Mg of the main component, and Al, Mn, and Ni to be added, respective metals may be blended so long as these can be dissolved (dispersed) by heating, or heating and stirring, and Ni may be added to an Mg—Al Mn alloy, an Mg—Al—Zn—Mn alloy, an Mg—Al—Mn—Ca alloy or an Mg—Al—Zn—Mn alloy.

The Mg alloy contains a crystallized Al—Mn—Ni system intermetallic compound. As described above, Ni has high melting point and high density, and hence is difficult to disperse in the Mg alloy alone. However, Ni forms an Al—Mn—Ni system intermetallic compound together with Al and Mn to be added into the Mg alloy for crystallization, and thereby is dispersed in the Mg alloy which can promote the degradation of the Mg alloy.

The inclusion of the crystallized Al—Mn—Ni system intermetallic compound in the Mg alloy has the effect of enhancing the degradation rate of the Mg alloy, the effect of being able to uniformly degrade the Mg alloy according to the site of crystallization, or the effect of being able to cause local degradation thereof. In terms of the enhancement of the degradation rate, the Mg alloy is “easily degradable”. The details of the Al—Mn—Ni system intermetallic compound will be described later.

The Al to be added to the Mg alloy is in an amount of preferably 0.1 mass % or more, more preferably 0.1 mass % or more and 16 mass % or less, and further preferably 0.1 mass % or more and 11 mass % or less, or 0.3 mass % or more and 11 mass % or less, based on the total amount of the Mg alloy. When Al is in an amount of less than 0.1 mass %, an Al—Mn—Ni system intermetallic compound becomes less likely to be formed. On the other hand, when Al increases in an amount, the internal stress during casting increases, so that continuous casting tends to become difficult. However, it is essential only that the amount of Al falls within the range of the density with which the formation and the crystallization of the Al—Mn—Ni system intermetallic compound are performed.

The Mn to be added to the Mg alloy is in an amount of preferably 0.05 mass % or more, more preferably 0.05 mass % or more and 1.0 mass % or less, and further preferably 0.1 mass % or more and 1.0 mass % or less based on the total amount of the Mg alloy. When Mn is in an amount of less than 0.1 mass %, an Al—Mn—Ni system intermetallic compound becomes less likely to be formed. On the other hand, when Mn increases in amount, Mn tends to become less likely to be contained in the Mg alloy, particularly, the Mg alloy containing Al. However, it is essential only that the amount of Mn falls within the range of the density with which the formation and the crystallization of the Al—Mn—Ni system intermetallic compound are performed.

The Ni to be added to the Mg alloy is preferably in an amount of 0.1 mass % or more based on the amount of the crystallized Al—Mn—Ni system intermetallic compound. When Ni is in an amount of less than 0.1 mass %, such a potential difference as to promote the degradation between the Al—Mn—Ni system intermetallic compound and α-Mg becomes less likely to be generated. On the other hand, when Ni increases in amount, the crystallization temperature of the Al—Mn—Ni system intermetallic compound becomes higher. For this reason, when crystallization starts in the Mg molten metal, precipitation is caused, and separation becomes more likely to be caused. However, it is essential only that the amount of Ni falls within a range of the density with which the formation and the crystallization of the Al—Mn—Ni system intermetallic compound are properly performed.

Incidentally, the Ni to be contained in the Mg alloy is preferably 0.01 mass % or more, more preferably 0.01 mass % or more and 0.6 mass % or less, and further preferably 0.01 mass % or more and 0.5 mass % or less, based on the total amount of the Mg alloy. This is conceivably because even when Ni is added in an amount of 0.6 mass % or more, the amount of Ni that is sedimented and separated at the furnace bottom without being sufficiently dispersed/diffused in the Mg alloy increases (see the results of the evaluation test 5 described later).

Further, the intermetallic compound is a compound formed of two or more kinds of metals, and is the one exhibiting distinctive physical and chemical properties different from those of the forming elements.

The Mg alloy may be allowed to further contain Zn. Zn to be added to the Mg alloy is preferably in an amount of 0.05 mass % or more and 1.5 mass % or less, and more preferably in an amount of 0.1 mass % or more and 1.5 mass % or less, based on the total amount of the Mg alloy. Addition of Zn to the Mg alloy can improve 0.2% proof strength and elongation, and can promote the aging precipitation due to solid solution strengthening. On the other hand, when Zn is added in an amount of more than 1.5 mass %, the degradation rate tends to decrease.

The Mg alloy may be allowed to further contain Ca. The Ca to be added to the Mg alloy is preferably in an amount of 0.1 mass % or more and 2.0 mass % or less, and more preferably in an amount of 0.2 mass % or more and 2.0 mass % or less. Addition of Ca to the Mg alloy to which Al has been added crystallizes one or more compounds selected from the group consisting of Al₂Ca, (Mg, Al)₂Ca, and Mg₂Ca. The compounds contribute as the driving force of degradation, resulting in an increase in degradation rate. Further, crystallization of the compounds can provide an Mg alloy improved in flame retardant property and heat resistant strength. Herein, the ratio of the compounds containing Ca is roughly determined by the addition ratios of Al and Ca. When the addition ratios are Al>Ca, Al₂Ca becomes the main. When the addition ratios are Al≈Ca, (Mg, Al)₂Ca becomes the main. When the addition ratios are Al<Ca, Mg₂Ca becomes the main.

On the other hand, when the Ca addition amount exceeds 2.0 mass %, the tensile characteristics such as 0.2% proof strength and elongation may be reduced.

The number density of the crystallized Al—Mn—Ni system intermetallic compounds present in the Mg alloy is preferably one compound/cm² or more per unit sectional area in SEM or a metal microscope. This is because the Al—Mn—Ni system intermetallic compounds which are noble in electric potential are preferably crystallized in an amount of one compound/cm² or more per unit sectional area for ensuring the degradation rate.

Further, the crystallized Al—Mn—Ni system intermetallic compound preferably has a size of 1 nm or more and 25 μm or less. This is because when the Al—Mn—Ni system intermetallic compound is crystallized with a particle diameter of 25 μm or more, the compound may become a starting point of breakage including fatigue (see the results of an evaluation test 5 described later). When the size of the Al—Mn—Ni system intermetallic compound which is noble in electric potential as described above is adjusted, the degree of promotion of degradation can be adjusted according to the intended purpose.

The crystallized Al—Mn—Ni system intermetallic compounds present in the Mg alloy are preferably present in a larger number at the crystal grain boundary than the number of the compounds present in the crystal grain interior in the extruded material after the extruding step shown in FIG. 1 . Namely, preferably, more than 50% and 100% or less intermetallic compounds of the total Al—Mn—Ni system intermetallic compounds are present at the crystal grain boundary. The Al—Mn—Ni system intermetallic compounds present at the crystal grain boundary are stable in a high temperature region, and hence has an effect (pinning effect) of stopping coarsening and growth of microscopic crystal grains of the Mg alloy formed by distortion caused by extrusion (plastic working). As a result, the crystal structure of the Mg alloy of the extruded material can be finely uniformalized, and degradation can also be made uniform.

Incidentally, in the billet subjected to the homogenization treatment before the extruding step (plastic working), the proportion of the number of compounds present in the crystal grain interior may be larger than that of the extruded material. Specifically, 30% or more and 100% or less intermetallic compounds of the total Al—Mn—Ni system intermetallic compounds may be present in the crystal grain interior.

Further, when an inclusion is assumed to be a heterogeneous nucleus for adding Ni to the Mg alloy containing Al, or in other cases, the Al—Mn—Ni system intermetallic compounds can become in shape of clusters. When the Al—Mn—Ni system intermetallic compounds are formed in shape of a clusters at the degradation surface of the contact surface with the solution, the area of the portion which is noble in electric potential increases, which can locally increase the degradation rate.

The Mg alloy may contain other elements than essential Mg, Al, Mn, and Ni, and arbitrary Zn and Ca, and other elements may be only inevitable impurities. Herein, examples of the inevitable impurities may include but are not limited to Si, Fe, and Cu. In other words, in the Mg alloy, the balance of essential Al, Mn, and Ni, and arbitrary Zn and Ca may be Mg and inevitable impurities.

Incidentally, the effects of respective elements are roughly as follows. Al promotes solid solution strengthening and precipitation strengthening, and improves the casting property and the corrosion resistance. Mn suppresses coarsening of the recrystallized crystal grains in plastic working. Zn improves the casting property and the strength. Ca improves the creep strength and the heat resistant strength, and imparts the flame retardant property.

(Method for Manufacturing Mg Alloy)

A method for manufacturing an Mg alloy of the present invention includes a casting step, a homogenization treatment step, an extruding step, or a forging step.

FIG. 1 shows a simplified flow of the method for manufacturing an Mg alloy. At the casting step, a billet is produced. For the billet, the billet subjected to the homogenization treatment at the homogenization treatment step is produced. For the billet subjected to the homogenization treatment, an extruded material is produced at the extruding step, or a forging material is produced at the forging step. Incidentally, the extruded material and the forging material are also referred to as plastic working materials.

(Casting)

The casting step includes a step of producing a mixture; a step of producing a molten metal by heating; a step of producing a completely molten metal by stirring; and a step of casing the completely molten metal.

The step of blending Mg, Al, Mn, and Ni, and producing a mixture at the casting step is a step of preparing ground metals or metal ingots according to the alloy composition, and mixing them, and producing a mixture.

Other than essential Mg, Al, Mn, and Ni, arbitrary Zn and Ca can be blended.

One example of the subsequent step of heating the produced mixture, and producing a molten metal is the step of heating the mixture to 720° C. or more, preferably 730° C. or 740° C., and more preferably 750° C. or more. Incidentally, in the case of a temperature as high as more than 750° C., the molten metal is rendered in an active state, so that a large number of vacancy defects may become more likely to be formed.

The step of stirring the produced molten metal, and producing a completely molten metal is a step of stirring a heated mixture, further dissolving the mixture evenly and nearly completely, and producing a completely molten metal. The completely molten metal denotes the one in a liquid state in which the blended ground metals and metal ingots, and the crystallized compounds are mixed evenly without precipitation or separation from the Mg alloy. Further, examples of stirring may include mechanical stirring, manual stirring, ultrasonic molten metal stirring, and electromagnetic stirring.

Although the stirring time is dependent upon the amount and the temperature of the heated molten metal, the stirring method, the size and the power of the stirring device, and the like, 10 minutes or more and 60 minutes or less is exemplified. When the Mg alloy is subjected to stirring at high temperatures for a long time, a large amount of film and oxides on the molten metal surface are wound, which may make it impossible to keep the ingot quality. In such a case, by adjusting the stirring time, or carrying out a molten metal treatment after stirring, and thereby performing adjustment so that the film and the oxides on the molten metal surface may not be included at least in the billet, it is possible to keep the quality of the billet, the billet subjected to the homogenization treatment, the extruded material, or the forging material (plastic working material).

The produced completely molten metal is poured into a die with a diameter of 70 (an internal diameter of 70 mm) as one example, so that a billet is produced.

Incidentally, casting in the casting step denotes raising the temperature of the metal up to the melting point or higher, pouring the metal into a die, and cooling and solidifying the metal. The casting method of the casting step of the present invention has no restriction so long as the method performs such casting. Examples thereof may include a sand mold casting method (such as a green (sand) mold casting method, a dry type casting method, a self-hardening mold casting method, a thermosetting mold casting method, a gas setting mold casting method, a sublimation pattern casting method, a V step casting method, or a frozen mold manufacturing method), a gypsum casting method, a precision casting method, a die casting method (a gravity casting method, a die casting method, a low pressure casting method, or a high pressure casting method), and a continuous casting method.

(Homogenization Treatment Step)

The homogenization treatment step is a step of incorporating the intermetallic compound crystallized at the casting step into α-Mg in a solid solution form, suppressing the segregation of the components, and forming an ingot less in fluctuation of the component density. For example, in an Mg—Al—Zn—Ni system alloy, the Mg—Al—Zn system intermetallic compound with a low melting point to be crystallized at the casting step is incorporated in a solid solution form into α-Mg, and a homogenization treatment is performed. Incidentally, when an extruding step is performed with the low-melting-point compounds left, cracking tends to be caused. Further, when the Mg—Al—Zn system intermetallic compounds are left, there is a risk of ignition. Accordingly, the homogenization treatment step is one of the steps to be carried out not only from the viewpoint of forming an alloy less in fluctuation of the component density but also from the viewpoint of keeping the mechanical strength so as to make cracking or the like less likely to be caused, and from the viewpoint of safety such as ignition.

As one example, a billet with a diameter of 70 mm is cut to a diameter of 60 mm (an outer diameter of 60 mm), and is subjected to a homogenization treatment at 400° C. to 420° C., preferably about 410° C., thereby producing a billet subjected to the homogenization treatment.

(Extruding Step)

Extrusion at the extruding step is the following method: a material (such as a billet subjected to a homogenization treatment) is placed in a pressure-proof container, and the material is applied with a pressure, thereby to be extruded from a die drilled in a prescribed cross sectional shape, and to be formed into a desired cross sectional shape.

At the extruding step of the present invention, as one example, a billet subjected to a homogenization treatment is extruded in a 300° C. to 410° C., and preferably about 400° C. atmosphere so as to have a diameter of 10 mm (an outer diameter of 10 mm), thereby forming an extruded material (plastic working material). The extruded material is further worked into a component or a member using an Mg alloy.

(Forging Step)

Forging at the forging step is a method in which a material is placed between a pair of upper and lower dies, and crushed by pressing to be worked into a desirable shape.

At the forging step of the present invention, as one example, a billet subjected to a homogenization treatment is pressed by the upper die and the lower die in a 300° C. to 410° C., preferably about 400° C. atmosphere, thereby forming a forging material (plastic working material). Alternatively, forging is performed so as to implement a round cast with an appropriate size such as a diameter of 10 mm (an outer diameter of 10 mm), a length of 200 mm to 300 mm, or the like, thereby forming a forging material (plastic working material). The forging material is further cut into a component or a member using an Mg alloy.

(Application of Mg Alloy)

An Mg alloy is applied to construction materials such as the members for a structure and the like, a vibration damping member, a sacrificial electrode material, a drilling member, an underground structure, and an underwater structure, a biomaterial, a medical material, and the like. Particularly, construction materials for use underground or underwater, a biomaterial for use in the body can eliminate the necessity of collection after use by the degradability of the Mg alloy.

EXAMPLES

Below, the results of the evaluation test including Examples of the present invention will be specifically described. However, the present invention is not limited to the Examples at all.

—Evaluation Test 1 (Production of Evaluating Sample and Measurement of Degradation Rate)—

Metals were added (blended) so as to achieve the contents described in Tables 1A, 1B, and 1C, and respective extruded materials were formed by the casting step, the homogenization treatment step, and the extruding step, resulting in evaluating samples 1 to 51. Each mass % of the metals of Tables 1A, 1B, and 1C is the proportion of the metal to be contained in each evaluating sample. The alloy kind is the description of the appellation defined according to ASTM, or the name in reference to the rule of the appellation of ASTM. For example, A is aluminum, Z is zinc, M is manganese, N is nickel, and X is calcium, and the subsequent numbers are those obtained by rounding mass % to one digit, followed by sequential arrangement.

Further, for each of the samples 1 to 51, the degradation rate was measured. The measurement of the degradation rate was performed in the following manner: a specimen whose weight (mg) had been measured was immersed in a 93° C. 2%-KCl aqueous solution for a given time, and was extracted, followed by drying, and was measured for the weight (mg); and the change in weight was confirmed. The value obtained by converting the decrease in mass into the value per a surface area (1 cm²) per day is the degradation rate (mg/cm²/day).

TABLE 1A Degradation rate 93° C.-2% Al Zn Mn Si Fe Cu Ni Ca Mg KCl Sample (mass (mass (mass (mass (mass (mass (mass (mass (mass (mg/cm²/ No. Alloy kind %) %) %) %) %) %) %) %) %) day) Sample1 AZ91 + 0.007Ni 9.1 0.64 0.25 0.020 0.001 0.001 0.007 — Bal. 58 Sample2 AZ91 + 0.01Ni 8.9 0.60 0.23 0.022 0.004 <0.0005 0.010 — Bal. 115 Sample3 AZ80 + 0.01Ni 8.3 0.58 0,22 0.025 0.001 <0.0005 0.012 — Bal. 113 Sample4 AZ80 + 0.01Ni 8.0 0.57 0.24 0.026 0.001 <0.0005 0.014 — Bal. 156.5 Sample5 AZ80 + 0.01Ni 8.6 0.64 0.21 0.028 0.001 <0.0005 0.016 — Bal. 261 Sample6 AZ80 + 0.01Ni 8.4 0.59 0.23 0.026 0.001 <0.0005 0.015 — Bal. 320 Sample7 AZ80 + 0.01Ni 8.8 0.59 0.24 0.022 0.001 0.001 0.018 — Bal. 501 Sample8 AZ80 + 0.02Ni 8.4 0.61 0.25 0.017 0.001 <0.0005 0.020 — Bal. 584 Sample9 AZ80 + 0.03Ni 8.4 0.60 0.25 0.017 <0.0005 <0.0005 0.027 — Bal. 737 Sample10 AZ80 + 0.04Ni 8.4 0.61 0.25 0.017 <0.0005 <0.0005 0.040 — Bal. 1017 Sample11 AZ80 + 0.05Ni 8.5 0.60 0.25 0.017 0.001 <0.0005 0.050 — Bal. 1031 Sample12 AZ80 + 0.08Ni 8.3 0.59 0.25 0.016 0.001 <0.0005 0.077 — Bal. 1310 Sample13 AZ80 + 0.3Ni 7.9 0.58 0.20 0.018 <0.0005 0.001 0.338 — Bal. 1242 Sample14 AZ80 + 0.5Ni 7.8 0.52 0.19 0.019 0.001 0.004 0.530 — Bal. 1441

TABLE 1B Degradation rate 93° C.-2% AI Zn Mn Si Fe Ca Ni Ca Mg KCl Sample (mass (mass (mass (mass (mass (mass (mass (mass (mass (mg/cm²/ No. Alloy kind %) %) %) %) %) %) %) %) %) day) Sample15 AXMN05020705 0.6 — 0.76 0.004 0.002 0.001 0.593 0.25 Bal. 1700 Sample16 AZXN405105 3.9 0.49 0,58 0.012 0.001 <0.0005 0.330 1.10 Bal. 1803 Sample17 AMXNG0205 5.7 — 0.31 0.010 0.006 <0.0005 0.461 2.16 Bal. 2500 Sample18 AZX801 + 0.009Ni 8.2 0.61 0.24 0.026 0.001 0.001 0.009 1.18 Bal. 227 Sample19 AZX801 + 0.01Ni 8.0 0.58 0.33 0.026 0.002 0.001 0.013 1.14 Bal. 430 Sample20 AZX801 + 0.02Ni 8.0 0.58 0.34 0.026 0.001 0.001 0.018 1.10 Bal. 715 Sample21 AZX801 + 0.03Ni 8.1 0.60 0.34 0.030 0.001 <0.0005 0.034 1.15 Bal. 1374 Sample22 AZX801 + 0.04Ni 8.0 0.62 0.34 0.026 0.002 0.001 0.045 1.21 Bal. 1645 Sample23 AZX801 + 0.07Ni 8.0 0.61 0.33 0.029 0.002 0.001 0.071 1.10 Bal. 2084 Sample24 AZX801 + 0.1Ni 7.9 0.59 0.34 0.030 0.001 <0.0005 0.097 1.03 Bal. 2002 Sample25 AMXNB00204 8.5 — 0.40 0.021 0.003 0.001 0.370 0.24 Bal. 2900 Sample26 AZXN800205 8.5 0.52 0.39 0.024 0.006 <0.0005 0.470 0.24 Bal. 2800 Sample27 AMXNB00305 8.4 — 0.40 0.017 0.008 0.001 0.500 0.34 Bal. 3000 Sample28 AZXN800505 8.4 0.55 0.43 0.023 0.007 <0.0005 0.550 0.56 Bal. 3600 Sample29 AMXN800505 8.6 — 0.38 0.016 0.004 0.001 0.430 0.57 Bal. 3100 Sample30 AMXN900501 9.5 — 0.40 0.021 0.003 <0.0005 0.129 0.61 Bal. 2400 Sample31 AMXN900502 8.8 — 0.41 0.017 0.004 <0.0005 0.280 0.64 Bal. 2600

TABLE 1C Degradation rate 93° C.-2% KCl Sample Al Zn Mn Si Fe Cu Ni Ca Mg (mg/cm²/ No. Alloy kind (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) day) Sample32 AMXN800702 7.5 — 0.26 0.012 0.003 <0.0005 0.212 0.76 Bal. 3000 Sample33 AMXN800701 7.8 — 0.24 0.011 0.016 <0.0005 0.111 0.76 Bal. 2000 Sample34 AMXN800703 8.0 — 0.29 0.011 0.013 <0.0005 0.359 0.77 Bal. 3000 Sample35 AMXNB0102 7.5 — 0.26 0.011 0.004 <0.0005 0.213 1.04 Bal. 2400 Sample36 AMXN801007 8.1 — 0.26 0.012 0.003 <0.0005 0.076 1.07 Bal. 1700 Sample37 AMXN80103 8.0 — 0.24 0.011 0.005 <0.0005 0.319 1.08 Bal. 3300 Sample38 AMXN901007 9.1 — 0.25 0.013 0.010 <0.0005 0.081 1.08 Bal. 1900 Sample39 AMXN90101 9.2 — 0.39 0.021 0.006 <0.0005 0.145 1.12 Bal. 2500 Sample40 AZXN90101 9.3 0.22 0.36 0.021 0.006 <0.0005 0.137 1.12 Bal. 2400 Sample41 AMXN90102 9.0 — 0.26 0.011 0.009 <0.0005 0.209 1.11 Bal. 2600 Sample42 AMXN90103 9.1 — 3,27 0.011 0.004 <0.0005 0.326 1.08 Bal. 2800 Sample43 AZXN805105 7.9 0.51 0.57 0.011 0.003 <0.0005 0.350 1.15 Bal. 3284 Sample44 AMXN80105 8.1 1 0.24 0.009 0.003 <0.0005 0.473 1.11 Bal. 3200 Sample45 AMXNS0205 8.2 — 0.24 0.009 0.003 <0.0005 0.451 2.14 Bal. 3900 Sample46 AZXN80205 7.7 0.51 0.66 0.016 0.004 <0.0005 0.370 2.30 Bal. 3121 Sample47 AMXN1107205 10.8 — 0.54 0.015 0.004 <0.0005 0.350 2.32 Bal. 3244 Sample48 AZN6104 5.6 0.61 0.25 0.018 <0.0005 <0.0005 0.400 0.00 Bal. 1300 Sample49 AZN6205 5.7 2.07 0.24 0.010 0.005 <0.0005 0.532 0.00 Bal. 700 Sample50 AZ31 2.9 0.86 0.41 0.017 0.002 0.001 0.001 — Bat. 4 Sample51 AZ80 8.0 0.59 0.24 0.031 0.002 0.001 0.001 — Bal. 6

—Evaluation Test 2 (Regarding Proportion of the Content of Ni Relative to Addition Amount of Ni)—

It is important for the present invention that the added Ni forms and crystallizes an Al—Mn—Ni system intermetallic compound in the Mg alloy. However, when the added Ni cannot form an Al—Mn—Ni system intermetallic compound in the Mg alloy sufficiently, or when the added Ni cannot be dispersed in the Mg alloy and is precipitated and removed due to the high melting point and the high density of Ni, the Ni content of the billet or the plastic working material (the extruded material or the forging material) becomes lower relative to the addition amount.

Thus, the proportion of the Ni content in the billet relative to the Ni addition amount according to the heating temperature (molten metal temperature), and with or without stirring at the casting step was measured.

The results are shown in FIG. 2 and Table 2.

TABLE 2 Proportion of Ni content relative Molten metal to Ni addition amount(%) temperature Without With (° C.) stirring stirring 700 2 8 710 4 7 720 3 21 730 15 27 740 26 91 750 9 100

As shown in FIG. 2 and Table 2, it has been indicated as follows: when the heating temperature becomes 720° C. or more, the proportion of the Ni content in the case with stirring becomes higher as compared with the case without stirring; when the heating temperature becomes 740° C. or more, in the case with stirring, 90% or more of the addition amount of Ni is blended with the billet or the plastic working material (the extruded material or the forging material).

Accordingly, it has been indicated as follows: under the conditions of the present evaluation test 2, when stirring is performed at a heating temperature of 720° C. or more, Ni is dissolved (dispersed) in the Mg alloy.

—Evaluation Test 3 (SEM-EDS Analysis by Ni Content)—

The extruded material without stirring and the extruded material with stirring at a molten metal temperature of 750° C. at the casting step of Table 2 were subjected to SEM-EDS analysis.

A SEM (scanning electron microscope) irradiates a target sample with an electron beam, and detects secondary electrons and the like emitted from the target sample, thereby analyzing the structure of the surface of the target sample. An EDS (Energy Dispersive X-ray Spectroscopy) detects the fluorescent X rays generated upon application of an electron beam or an X ray to the target sample, thereby analyzing the elements forming the target sample, and the density thereof.

The structure of the surface of the extruded material according to with or without stirring at the casting step was confirmed by an SEM, thereby analyzing the elements of the crystal portion and the density thereof by EDS.

FIG. 3(a) shows the visual field of the extruded material without stirring at a heating temperature (molten metal temperature) of 750° C., and Table 3 shows the point analysis results. Further, FIG. 3(b) shows the visual field of the extruded material with stirring at a heating temperature (molten metal temperature) of 750° C., and Table 4 shows the point analysis results.

TABLE 3 Unit (mass %) Position Mg Al Mn Zn Ni 001 90.93  7.51 1.56 — — 002 64.11 26.77 9.12 — — 003 84.36 13.80 1.08 0.76 — 004 71.54 26.50 — 1.96 —

TABLE 4 Unit (mass %) Position Mg Al Mn Zn Ni 001 29.82 50.25 13.08 6.85 002 70.73 17.48  7.01 4.79 003 73.23 25.40 — 1.37 — 004 82.14 16.97 — 0.89 —

From the analysis results of the positions 001 and 002 of FIG. 3(a) and Table 3, for the extruded material without stirring at 750° C., the formation of the Al—Mn—Ni system intermetallic compound was not observed.

On the other hand, from the analysis results of the positions 001 and 002 of FIG. 3(b) and Table 4, for the extruded material with stirring at 750° C., the formation of the Al—Mn—Ni system intermetallic compound was observed.

Accordingly, it has been indicated as follows: by performing stirring, thereby dissolving and dispersing Ni with more reliability, the Al—Mn—Ni system intermetallic compound is formed and crystallized.

—Evaluation Test 4 (Production and Evaluation of Billet)—

Regarding the production of the billet at the casting step, FIG. 4(a) shows one example of the elapsed time, the temperature, and the execution step of the casting step, FIG. 4(b) shows a metal microphotograph of the billet casted at FIG. 4(a), and FIG. 4(c) shows a metal microphotograph on an enlarged scale of FIG. 4(b).

At the casting step, the step of performing heating and stirring in FIG. 4(a) is the step for dissolving Ni in the Mg alloy evenly and completely or nearly completely, and producing a completely molten metal. Before and/or after the step of performing stirring, a molten metal treatment step can be carried out. The molten metal treatment step is the step for performing heating and stirring, thereby keeping the ingot quality which tends to be degraded.

For the billet which had gone through the step of performing heating and stirring as in FIG. 4(a), and the molten metal treatment step, such metal microphotographs as FIGS. 4(b) and 4(c) were obtained, and the formation and the crystallization of the Al—Mn—Ni system intermetallic compound were observed by EDS analysis.

Accordingly, also from the results of the evaluation test 4, it has been indicated as follows: as with the evaluation test 3, heating and stirring are performed, thereby completely dissolving and dispersing Ni with more reliability; as a result, the Al—Mn—Ni system intermetallic compound is formed and crystallized.

—Evaluation test 5 (regarding addition amount of Ni)—

In the evaluation test 2, it has been indicated that the proportion of the Ni content in the billet relative to the addition amount of Ni at the casting step can be increased by stirring at the casting step. On the other hand, the crystallization temperature of the Al—Mn—Ni system intermetallic compound tends to increase with an increase in addition amount of Ni. For this reason, even when stirring is performed, the Al—Mn—Ni system intermetallic compound may not be formed sufficiently. Thus, the billets were evaluated according to the difference in addition amount of Ni.

When the Ni addition amount was 0.4 mass %, the Ni content of the billet was 0.4 mass %. FIG. 5(a) shows the SEM-EDS analysis results of the billet.

It could be observed that the billet contained Ni added in an amount of 100% (content/addition amount=0.4/0.4=100%, and that the Al—Mn—Ni system intermetallic compounds were formed, and were present in a shape of a needle or a grain at the crystal grain boundaries. The reason why an intermetallic compound in a shape of a needle or a grain is formed at the crystal grain boundary can be considered as follows. The Al—Mn—Ni system metal compound exerts the pinning effect with respect to coarsening of recrystallized crystal grains for plastic working, and thereby suppresses coarsening of the recrystallized crystal grains.

On the other hand, when the Ni addition amount was 5 mass %, the Ni content of the billet was 0.4 mass %. FIG. 5(b) shows the SEM-EDS analysis results of the billet.

The billet contained Ni added in an amount of only 8% (content/addition amount=0.4/5=8%). It could be confirmed that an Al—Mn—Ni system intermetallic compound was crystallized in a shape of dendrite (dendric shape) in the billet. The reason why the intermetallic compound in a shape of dendrite is formed is considered as follows: the crystallization temperature is higher than that of α-Mg, and the compound is formed as a primary crystal compound. Further, also from the present evaluation test results, it is considered as follows: even when Ni is added in an amount of 0.6 mass % or more in the Mg—Al—Zn—Mn system alloy, it is not sufficiently dispersed/diffused in the Mg alloy, and is precipitated and separated to the furnace bottom.

Accordingly, it has been indicated as follows: when the proportion of the Ni content relative to the addition amount of Ni becomes too low, the Al—Mn—NI system intermetallic compound cannot be sufficiently dispersed in the Mg alloy, a coarse Al—Mn—Ni system intermetallic compound with a particle diameter of 25 μm or more is crystallized. Incidentally, when the Al—Mn—Ni system intermetallic compound is crystallized with a particle diameter of 25 μm or more, it may become a starting point of breakage including fatigue.

—Evaluation Test 6 (Crystallization Position of Al—Mn—Ni System Intermetallic Compound in Billet Subjected to Homogenization Treatment)—

After the homogenization treatment step, the crystallization position of the Al—Mn—Ni system intermetallic compound in the billet subjected to a homogenization treatment before the extruding step was evaluated.

A billet subjected to a homogenization treatment planned to contain Ni in an amount of 0.4 mass % in AZ80 (an Mg alloy planned to contain Al in an amount of 8 mass % and Zn in an amount of 0% in terms of rounded value) was produced, and was subjected to metal microscope observation.

FIG. 6(a) shows a metal microphotograph of a visual field 1, FIG. 6(b) shows a metal microphotograph of a visual field 2, and FIG. 6(c) shows a metal microphotograph of a visual field 3. The results obtained by counting the number of the compounds present in a crystal grain interior and the number of the compounds present at the crystal grain boundaries for the Al—Mn—Ni system intermetallic compounds are shown in Table 5.

TABLE 5 Al—Mn—Ni Visual Visual Visual Total [number] field 1 field 2 field 3 amount Grain interior 40 38 40 118 Grain boundary 18 6 15 39 Total number 58 44 55 157 Grain interior/ 699% 86% 73% 75% total number[%] Grain boundary/  31% 14% 27% 25% total number[%]

FIGS. 6(a), 6(b), and 6(c), and Table 5 indicates that about 65% to about 90% of the Al—Mn—Ni system intermetallic compounds of the billet subjected to a homogenization treatment before the extruding step at the present evaluation test are present in a crystal grain interior.

—Evaluation Test 7 (Crystallization Position of Al—Mn—Ni System Intermetallic Compound in Extruded Material)—

The crystallization position of the Al—Mn—Ni system intermetallic compound in the extruded material after the extruding step was evaluated.

A billet subjected to a homogenization treatment planned to contain Ni in an amount of 0.4 mass % in AZ80 (an Mg alloy planned to contain Al in an amount of 8 mass % and Zn in an amount of 0% in terms of rounded value) was produced, and was subjected to metal microscope observation.

FIG. 7(a) shows a metal microphotograph of a visual field 1, and FIG. 7(b) shows a metal microphotograph of a visual field 2. The results obtained by counting the number of the compounds present in a crystal grain interior and the number of the compounds present at the crystal grain boundaries for the Al—Mn—Ni system intermetallic compounds are also shown along with FIGS. 7(a) and 7(b).

FIGS. 7(a) and 7(b) indicate that about 80% to about 85% of the Al—Mn—Ni system intermetallic compounds of the extruded material after the extruding step in the present evaluation test are present at the crystal grain boundaries.

The ratio of the crystallization positions of the Al—Mn—Ni system intermetallic compounds varies between before and after the extruding step. This is roughly as follows.

When the extruding step (plastic working) is performed, distortion is caused in the Mg alloy, resulting in the formation of microscopic crystal grains. The crystal grains are coarsened for recovery of the distortion. However, the presence of such compounds as the stable Al—Mn—Ni system intermetallic compounds in the high-temperature region causes the pinning effect of suppressing the coarsening (growth) of crystals to act. The pinning effect occurs at the grain boundaries between crystal grains. For this reason, the Al—Mn—Ni system intermetallic compounds come to be present at the crystal grain boundaries. Further, by pinning coarsening of crystals, the crystals in the inside of the extruded material become microscopic and uniform, and become stable.

Accordingly, it is indicated that the Al—Mn—Ni system intermetallic compounds present at the crystal grain boundaries between microscopic and uniform crystals are finely dispersed in the Mg alloy, and can promote the degradation of the Mg alloy, and tends to control the degradation rate with ease evenly throughout.

—Evaluation Test 8 (Regarding Relationship Between Ni Density and Degradation Rate, and Whether Ca is Added or not)—

From among the evaluating samples of Tables 1A, 1, and 1C, the relationship between the Ni density and the degradation rate of each extruded material was evaluated according to whether Ca is added or not.

FIG. 8 shows the evaluation results.

FIG. 8 indicates as follows: first, when the content of Ni increases, the degradation rate increases. Further, when Ca was added, the degradation rate more increased than when Ca was not added, and the degradation rate became about 2.0 times when Ni was in an amount of 0.2 mass %, about 2.2 times when Ni was in an amount of 0.4 mass %, and about 2.4 times when Ni was in an amount of 0.6 mass %.

Accordingly, it has been indicated as follows: from the viewpoint of the degradation rate of the Mg alloy, the degradation rate can be increased when Ca is added.

—Evaluation Test 9 (SEM-EDS Analysis in Ca-Added Mg Alloy)—

In the Al-added Mg alloy (AZ system alloy, sample 28 in Table 1), when Ca is added, mainly Al₂Ca is formed when Al>Ca; mainly (Mg, Al)₂Ca is formed when Al≈Ca; and mainly Mg₂Ca is formed when Al<Ca.

FIGS. 9(a) and 9(b), and Tables 6 and 7 show each example of the SEM-EDS analysis results of the case where Ca is added (sample 43 of Table 1C) in the Mg alloy added with Al. FIG. 9(a) and Table 6 show a visual field 1, and FIG. 9(b) and Table 7 show a visual field 2.

TABLE 6 Unit (mass %) Position Mg Al Ca Mn Ni Zn 001 95.14 4.42 — 0.06 0.24 0.14 002 95.40 4.39 — — 0.01 0.20 003 95.38 4.53 0.01 — 0.01 0.07 004 94.94 4.83 0.00 — 0.12 0.11 005 95.40 4.38 0.02 0.04 0.05 0.12 006 9.76 48.50 0.02 12.18 29.54 — 007 5.94 52.95 0.11 23.82 16.93 0.24 008 76.69 22.05 0.83 — 0.05 0.37 009 83.05 16.32 0.47 — — 0.16 010 68.49 28.63 2.16 — 0.10 0.62

TABLE 7 Unit (mass %) Position Mg Al Ca Mn Ni Zn 001 96.00 3.47 0.05 — 0.05 0.43 002 96.50 3.27 0.01 0.03 0.07 0.11 003 95.83 3.74 0.07 0.02 0.05 0.29 004 95.82 3.66 0.04 0.06 0.05 0.36 005 96.29 3.32 — 0.06 0.03 0.30 006 68.59 10.82 0.25 10.17 9.05 1.13 007 92.46 6.52 0.47 0.08 0.16 0.32 008 96.53 3.12 0.09 — 0.07 0.19 009 92.58 5.38 1.36 0.07 0.05 0.56 010 92.30 6.95 0.27 0.01 0.13 0.34 0.11 96.30 3.31 0.02 — 0.03 0.35

Further, the degradation rate of the Mg alloy added with Ca (sample 43 of Table 1C) was about 3000 mg/cm²/day, and became about twice the degradation rate of the Mg alloy not added with Ca (see FIG. 8 ) of about 1500 mg/cm²/day.

Accordingly, also from the present evaluation test results, it has been indicated that, from the viewpoint of the degradation rate, addition of Ca can increase the degradation rate.

—Evaluation Test 10 (Relationship Between Ca Addition Density, and Characteristics of Tensile Rupture Strength, 0.2% Proof Strength, and Elongation)—

As shown in the evaluation tests 8 and 9, it has been indicated that addition of Ca can increase the degradation rate. Herein, other characteristics were also evaluated.

Metals were added (blended) so as to achieve the contents shown in Table 8, and each extruded material was formed by the casting step, the homogenization treatment step, and the extruding step, resulting in evaluating samples 52 to 55 (AM90+Ni mass %).

TABLE 8 Sample Al Zn Mn Si Fe Cu Ni Ca Mg No. Alloy kind (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) Sample52 AMN900 9.0 — 0.32 0.005 0.002 <0.0005 0.105 — Bal. Sample53 AMXN9000 9.5 — 0.40 0.021 0.003 <0.0005 0.129 0.606 Bal. Sample54 AMXN9010 9.2 — 0.39 0.021 0.006 <0.0005 0.145 1.12 Bal. Sample55 AMXN8020 8.2 — 0.24 0.009 0.003 <0.0005 0.451 2.14 Bal.

The sample 52) to 55 of table 8 were subjected to a tensile test according to JIS Z224 (metal material tensile testing method), and the measurements of the tensile rupture strength, the 0.2 proof strength, and the elongation were performed. Incidentally, for the tensile test specimen shape, JIS 14A specimen was adopted.

The measurement results are shown in Table 9 and FIG. 10 .

TABLE 9 Alloy Tensile rupture 0.2% proof Elongation Sample No. kind strength (Mpa) strength (Mpa) (%) Sample52 AMN900 332 246 13 Sample53 AMXN9000 315 233 9 Sample54 AMXN9010 312 226 9 Sample55 AMXN8020 294 205 10

FIG. 10 indicates that an increase in content of Ca remarkably reduces the 0.2% proof strength and the elongation.

Accordingly, an increase in Ca content increases the degradation rate of the Mg alloy from the evaluation test 8, or the like. However, it has been indicated as follows: at least from the viewpoints of the 0.2% proof strength and the elongation, the characteristics may be reduced; in consideration of this, the addition amount is required to be adjusted according to the intended purpose.

—Evaluation Test 11 (Consideration of Degradation Mechanism)—

An extruded material of AZ80+0.1 Ni alloy (sample 44 of Table 1C) was embedded in a resin, and a structure observation was performed. Then, the sample was immersed in a 93° C. 2% KCl solution, thereby performing structure observation at the same position 8 minutes later.

FIG. 11(a) shows the extrusion direction and the observation direction of the sample, FIG. 11(b) shows a metal microphotograph of the extruded material before the immersion test, and FIG. 11(c) shows a metal microphotograph of the extruded material after the immersion test.

The white part of FIG. 11(b) is α-Mg, and the densities of Al and Ca forming the alloy are relatively lower. Further, the black band-shaped portion of FIG. 11(b) is a 8 phase and Al₂Ca compounds, and the densities of Al and Ca are relatively higher. Further, the portion looking granular of FIG. 11(b) is an Al—Mn—Ni system intermetallic compound. The order of the electric potentials thereof is Al—Mn—Ni system intermetallic compound>Al₂Ca>β phase>α-Mg, and the α-Mg is the most base part.

On the other hand, as shown in FIG. 11(c), it has been indicated that the white portion, namely the α-Mg has been degraded. In other words, not the periphery of the Al—Mn—Ni system intermetallic compound which is noble in electric potential but the α-Mg of the base portion in electric potential has been degraded.

From the results up to this point, it has been considered as follows: the degradation reaction is not the galvanic reaction being locally effected mainly at the Al—Mn—Ni system intermetallic compound, but the degradation mechanism in which the galvanic reaction is effected macroscopically in plane, and degradation proceeds preferentially from the α-Mg of the most base portion in electric potential.

—Evaluation Test 12 (Cluster Formation of Al—Mn—Ni System Intermetallic Compound)—

Due to setting of the inclusion as the heterogeneous nucleus upon adding Ni to the AZ system alloy, or other reasons, the Al—Mn—Ni system intermetallic compound is crystallized in cluster shapes.

FIGS. 12(a) and 12(b) each show a metal microphotograph of the cluster formation.

When the clusters of the Al—Mn—Ni system intermetallic compound are present at the contact surface with the solution, the area of the portion noble in electric potential increases, and a large degradation rate can be obtained locally.

INDUSTRIAL APPLICABILITY

As described up to this point, the Mg alloy can ensure prescribed mechanical properties for a certain period. However, after an elapse of the period, the overall degradation rate can be controlled so as to allow dissolution or degradation. The Mg alloy of the present invention is expected to be applied to a Mg alloy for promoting degradation under various environments from the perspective of crystallization of the Al—Mn—Ni system intermetallic compound. 

1. A Mg alloy containing Mg, Al, Mn, and Ni, and having a crystallized Al—Mn—Ni system intermetallic compound.
 2. The Mg alloy according to claim 1, further containing Zn.
 3. The Mg alloy according to claim 1, further containing Ca.
 4. The Mg alloy according to claim 3, comprising one or more compounds selected from a group consisting of Al₂Ca, (Mg, Al)₂Ca, and Mg₂Ca.
 5. The Mg alloy according to claim 1, wherein the Al is in an amount of 0.1 mass % or more, and the Mn is in an amount of 0.05 mass % or more based on a total amount of the Mg alloy.
 6. The Mg alloy according to claim 2, wherein the Zn is in an amount of 0.05 mass % or more and 1.5 mass % or less based on a total amount of the Mg alloy.
 7. The Mg alloy according to claim 3, wherein the Ca is in an amount of 0.1 mass % or more and 2.0 mass % or less based on a total amount of the Mg alloy.
 8. The Mg alloy according to claim 1, wherein Ni is in an amount of 0.1 mass % or more based on an amount of the Al—Mn—Ni system intermetallic compound.
 9. The Mg alloy according to claim 1, wherein the Al—Mn—Ni system intermetallic compounds are present at one compound/cm² or more per unit sectional area, and has a size of 1 nm or more and 25 μm or less.
 10. (canceled)
 11. A method for manufacturing an Mg alloy having a crystallized Al—Mn—Ni system intermetallic compound, the method comprising casting, including: a step of producing a mixture by blending Mg, Al, Mn, and Ni; a step of producing a molten metal by heating the produced mixture to 720° C. or more; a step of producing a completely molten metal by stirring the produced molten metal; and a step of casting the completely molten metal produced by stirring.
 12. The method for manufacturing an Mg alloy according to claim 11, wherein the step of producing the mixture further includes a step of blending Zn and/or Ca.
 13. A construction material or a biomaterial each using the Mg alloy according to claim 1, wherein collection after use is unnecessary due to degradability of the Mg alloy.
 14. A construction material or a biomaterial each manufactured by the method for manufacturing an Mg alloy according to claim 11, wherein collection after use is unnecessary due to degradability of the Mg alloy.
 15. The Mg alloy according to claim 2, further containing Ca.
 16. The Mg alloy according to claim 15, comprising one or more compounds selected from the group consisting of Al₂Ca, (Mg, Al)₂Ca, and Mg₂Ca.
 17. The Mg alloy according to claim 2, wherein the Al is in an amount of 0.1 mass % or more, and the Mn is in an amount of 0.05 mass % or more based on a total amount of the Mg alloy.
 18. The Mg alloy according to claim 3, wherein the Al is in an amount of 0.1 mass % or more, and the Mn is in an amount of 0.05 mass % or more based on a total amount of the Mg alloy.
 19. The Mg alloy according to claim 15, wherein the Al is in an amount of 0.1 mass % or more, and the Mn is in an amount of 0.05 mass % or more based on a total amount of the Mg alloy.
 20. The Mg alloy according to claim 15, wherein the Ca is in an amount of 0.1 mass % or more and 2.0 mass % or less based on a total amount of the Mg alloy.
 21. The Mg alloy according to claim 2, wherein Ni is in an amount of 0.1 mass % or more based on the amount of the Al—Mn—Ni system intermetallic compound.
 22. The Mg alloy according to claim 3, wherein Ni is in an amount of 0.1 mass % or more based on a amount of the Al—Mn—Ni system intermetallic compound.
 23. The Mg alloy according to claim 15, wherein Ni is in an amount of 0.1 mass % or more based on an amount of the Al—Mn—Ni system intermetallic compound. 